Columbium base alloy

ABSTRACT

A COLUMBIUM BASE ALLOY IS DISCLOSED HAVING HIGH STRENGTH AND OXIDATION RESISTANT PROPERTIES PARTICULARLY AT ELEVATED TEMPERATURES AND ALSO ADEQUATE LOW TEMPERATURE DUCTILITY. THE ALLOY CONTAINS 20 TO 33% HAFNIUM, 11 TO 20% TUNGSTEN AND 0.5 TO 2% ZIRCONIUM, AND MAY CONTAIN SMALL AMOUNTS OF ONE OR MORE OTHER ELEMENTS INCLUDING ALUMINUM, IRON, CARBON AND TITANIUM TO MODIFY THE PROPERTIES OF THE ALLOY FOR VARIOUS APPLICATIONS, AND MAY ALSO CONTAIN UP TO ABOUT 4% OF TANTALUM WITHOUT MATERIALLY EFFECTING THE PROPERTIES OF THE ALLOY EXCEPT TO INCREASE ITS DENSITY.

United' States Patent 01 ice 3,667,940 Patented June 6, 1972 Teledyne,Inc., Albany, 'Oreg. No Drawing.Filed Apr. 10, 1969, Ser. No. 815,219

Int. (:1. C22c 27/00 us. or. 75-174 9 ClalmS ABSTRACT OF THE DISCLOSUREA columbium base alloy is disclosed having high strength and oxidationresistant properties particularly at elevated temperatures and alsoadequate low temperature ductility. The alloy contains 20 to 33%hafnium, 11 to 20% tungsten and 0.5 to 2% zirconium, and may containsmall amounts of one or more other elements including aluminum, iron,carbon and titanium to modify the properties of the alloy for variousapplications, and may also contain up to about 4% of tantalum withoutmaterially effecting the properties of the alloy except to increase itsdensity.

BACKGROUND OF INVENTION The invention relates to an alloy which has bothhigh strengths and good stress rupture properties as well as highresistance to oxidation at elevated temperatures and at the same timehas sutficient low temperature ductility to enable them to befabricated.

It is difiicult to obtain all of the properties referred to above. Prioralloys for use under stress at elevated temperatures have usually beendeficient in one or more of the desired properties. Thus, for example,it has been necessary to deleteriously sacrifice oxidation resistancefor high tensile strength or acceptable stress rupture properties orvice versa. In this application the term elevated temperatures isintended to mean temperatures above red heat, i.e. temperatures rangingfrom about 900 up to about 2400 I In accordance with the presentinvention an alloy containing between about 20 to 33% hafnium, about 11to 20% tungsten, and about 0.5 to 2% zirconium by weight with theremainder essentially columbium provides a balanced combination of thedesired properties and, in general, has greater oxidation resistancethan previous columbium base alloys. This basic'alloy may be modified bythe addition of small amounts of other elements to enhance certain ofthe properties of the alloy.

The hafnium in the columbium base alloy of the present inventionprovides high oxidation resistance and increased tensile strength atelevated temperatures when present in an amount at least equal to about20%. An increase in the hafnium content above about 33%, however, tendsto cause the tensile strength at elevated temperatures to againdecrease. Tungsten also provides increased strength at elevatedtemperatures and is employed in amounts at least equal to about 11% butan increase in tungsten content above about 20% decreases lowtemperature ductility. The zirconium also increases high temperaturestrength and oxidation resistance when employed in amounts at leastequal to about 0.5%. However, an increase in zirconium content aboveabout 2% causes the alloy to become brittle and difiicult to fabricate.

The outstanding properties of the alloy can be characterized as that ofhigh oxidation resistance and toughness throughout a range oftemperatures from room temperatures to at least 2400 F., the termtoughness being employed to mean a combination of high tensile strength,high impact strength and ductility. This toughness persists even in therange of 1200 to 1600 F. at which columbium base alloys are usuallydeficient in ductility. Also the stress rupture properties are adequatefor all usual purposes.

It is therefore an object of the invention to provide an improved hightemperature alloy having not only resistance to oxidation and excellentstress rupture properties at elevated temperatures but also having acharacteristic toughness including high tensile and impact strengths andductility over a range of temperatures from room temperature and belowup to about 2400 F.

V DESCRIPTION OF PREFERRED EMBODIMENTS The amounts of the elements inthe various forms of the alloy disclosed herein are given in percent byweight.

A general purpose alloy may, for example, contain 28% hafnium, 17%tungsten and 1.2% zirconium, the balance being essentially columbium. Analloy approximating this composition Will, in general have roomtemperature ductility of the order of 20% elongation with yieldstrengths of the order of 130,000 p.s.i. and ultimate strengths of140,000 p.s.i. At 1400 F. the ductility as measured by elongation isgreater than 10% with a yield strength of 80,000 p.s.i. and an ultimatestrength of 85,000 p.s.i. or greater and at 2400 F. the yield strengthis of the order of 35,000 p.s.i. and an ultimate strength of 40,000p.s.i. and ductility of the order of 50% elongation. The oxidationresistance of the alloy will, in general, be sufiiciently great that thesurface recession will be less than .006" and the oxide layer thicknessless than .035" when the alloy is heated in a current of air at 2400 F.for 24 hours.

As indicated above each of the various properties of the new alloy canbe materially enhanced by additions of small amounts of other elementswhile retaining the other properties at acceptable values.

It should first be pointed out, however, that tantalum frequently occursas an impurity in columbium and and is difficult to remove therefrom. Itcan be tolerated in any of the various modifications of the alloy up to'about 4% and, in general, can be considered a diluent which reduces theamount of tungsten which can be employed in an alloy of given density,thus decreasing high temperature strength. It does provide a smallimprovement in stress rupture properties and toughness but not enough tojustify its deliberate addition to the alloy. It, however, increases thedensity of the alloy and if a light weight metal is required, anyappreciable amount of tantalum is undesirable.

Of the other elements referred to above, the addition of aluminumimproves oxidation resistance at temperatures up to and including 2400F. An amount of aluminum in excess of about 1% deleteriously impairs lowtemperature ductility and tensile strength at elevated temperatures aswell as stress rupture properties at 2200 to 2400 F. The optimum amountof aluminum is about 0.2% to 0.25%.

The addition of small amounts of iron improves stress rupture propertiesat 2200 to 2400 F. Additions of iron in amounts above about 0.15% tendto cause the alloy to be brittle. The optimum amounts of iron forimprovement of stress rupture properties is about 0.07%.

The addition of small amounts of carbon prow'des a heat treatable alloy,the carbon apparently going into solution above 3000 F. andprecipitating out as finely dispersed complex carbides of the variousmetals upon cooling, the precipitated carbide being primarily hafniumcarbide. The addition of carbon provides increased tensile strength atelevated temperatures as high as 2400 F. as well as low temperatureductility and increases toughness throughout the temperature range ofroom temperature to 2400 F. Increasing the carbon content above andstress rupture properties when the alloy is proc essed to provide thebest low temperature ductility. Excess carbon also impairs oxidationresistance. The optimum amount of carbon for the purposes describedabove is about 0.1%.

The addition of titanium to the alloy improves fabricability at lowtemperatures or a few hundred degrees above room temperatures. It alsoimproves oxidation resistance particularly when relative large amountsof carbon are present in the alloy and increases yield strength up toabout 1400 F. It also reduces the density of the alloy but additions oftitanium in excess of about 5% deleteriously decreases stress ruptureproperties.

It will be apparent that the range of compositions of the alloy of thepresent invention is about as follows:

In almost all instances the composition of the alloy of the presentinvention will fall within approximately the following ranges:

CbBalance Feto 0.15% Hf.25 to 32% 0-0 to 0.15% W13 to 20% Ti-O to Zr-0.7to 1.8% Ta-0 to 4% Al-O to 0.5%

The composition of the alloy exhibiting the most desirable balance ofproperties will, in general, fall within approximately the followingpreferred ranges:

CbBalance Fe-0 to 0.07% Hf28 to 30% C-() to 0.1% W13 to 17% Ti0 to 5%Zr-0.7 to 1.8% Ta0 Al-O to 0.25%

As indicated above the various properties of the alloy can be enhancedfor the particular applications of the alloys. For example, gas turbineblades such as those employed in jet engines must withstand high tensilestresses and have high oxidation resistance at elevated and intermediatetemperatures and musthave excellent stress rupture properties as well aslow temperature ductility. An example of the composition of the alloywhich has about the lower acceptable limit of stress rupture propertiesis as follows:

Example I CbBalance Fe--0 Hf28% C-0 W-15.1% Ti--0 Zr-1.2% Ta3 .5 Al-0The alloy having this composition had a stress rupture life of 52.5hours at 2200 F. at 20,000 p.s.i. stress. This is substantially thebasic alloy containing Hf 28%, W 17% and Zr 1.2% first referred toherein except for the 3.5% tantalum and lower amount of tungsten. If thetantalum had not been inthe coluinbium so as to appear in the finalalloy, the tungsten percentage could have been raised to approximately17%. to provide greater high temperature tensile strength. Nevertheless,the alloy had excellent oxidation resistance and toughness includinggood high temperature strengths and intermediate and lower temperatureductility.

Another example of composition of an alloy having higher stress ruptureproperties is as follows: A

Example I! CbBalance ;Fe 0.05% Hf28% C0.13% W14.6% Ti-O Zr-l.2%:Ta.-0.1% (residual) Al-O' 1:

This example also had excellent oxidation resistance properties andtoughness at temperatures from room temperature to 2400 F. and had astress rupture life at 2400 F. and 20,000 p.s.i. from two to three timesthat of a similar alloy in which no iron was present.

From a consideration of the compositions and properties AlloyComposition A Q Gas turbine vanes are not-subjected to as high stressesas turbine blades and it is therefore possible to enhance the oxidationresistant properties 'to a somewhat greater extent than in, an alloy forturbine blades. An example of the composition of the alloy whichexhibited higher oxidation resistance with acceptable values of otherproperties primarily due to the addition of. aluminum is as follows:

' Example III CbBalance Fe-O A Hf30% C0.28% W--13% Ti,0 Zr-1.1% Ta--3.7%Al -0.25%

When oxidized at 2400 in air for hours, the surfacerecessio'n. wasapproximately .002" with an oxide, layer thickness of .035". Thisexample exhibited a yield tensile strength'at 3%elong'ation at roomtemperature after having been annealed at 2400 F. for 6 hours of 132,000p.s.i. and an ultimate tensile strengthof 136,- 000 p.s.i. At 1400 F.the yield tensile strength .at 3% elongation was 78,700 p.s.i. and theultimate tensile strength 93,400. p.s.i. The, stressrupture life, afterannealing at 2400 F. for onehour and measured at 2200 and 20,000p.s.ifwas 11.6,hours 'with a 36% elongation. The.

'1 plus 2400 Rim 6 hoursandmeasured at 2000 F. and

20,000 p.s.i. was 138.9 hours with 22.3% elongation.

Although this example of the. alloy is acceptable for turbine vanes,studies of other alloys show that a lower carbon content will increasethe stress rupture life and 7 this. reduction. will a1so increase"oxidation .resistance.

' Again the tantalum was presentin the materials from which the .alloywas made with consequent reduction of tungsten which could beadvantageously employed.

Anotherexample of the compositionof an alloy which canheemployed forvturbine vanes is as follows:

oxide layer thic kness of .023". After annealing at 2400" F. for 6,hours, ithad a. yield tensile strength at room Percent Composl-Composition B tion Balance Balance 28 28 15 15 1. 2 1. 2 0 0. 25 0 0 0.1 0. 1 5 0 0 0 The carbon in both of these compositions is at theapproximate optimum value of 0.1% to provide an alloy of the greatesttoughness throughout a temperature range from room temperature to 2400F. with very good oxidation resistance. The difference between thesecompositions is that composition C contains 0.25% aluminum andcomposition B contains 5% titanium, both of which increase oxidationresistance, and low temperature tensile strengths, the titanium beingparticularly effective when carbon is present in the alloy. The titaniumalso decreases the density of the alloy and improves fabricability.

The requirements for turbine blades and turbine vanes are similar andthe same composition of the alloy can be employed for both purposes, ifthe stress rupture properties are adequate for turbine blades. Thus thecomposition of an alloy which can be employed for both turbine bladesand turbine vanes is as follows:

Example V CbBalance Fe0' Hf-29% C0.1% W14.5% Ti-0 Zr1.1% Ta-3.7 Al-OThis example has exhibited a stress rupture life in excess of 30 hourswhen tested at 2200 F. under a 20,000 p.s.i. after annealing at 3400 F.for 2 hours plus 2400 F. for 6 hours and has exhibited room temperatureductility up to 33% elongation after annealing at 2400 F. for 6 hours.The alloy having this composition, when annealed at 2400 for 6 hours,also developed a yield tensile strength of 124,000 p.s.i. and anultimate tensile strength of 133,000 p.s.i. at room temperature, and ayield tensile strength of 70,900 p.s.i. and an ultimate tensile strengthof 93,600 p.s.i at 1400 F. It also had excellent oxidation resistanceproperties.

The tantalum content can be eliminated from the composition of Example Vand the tungsten content increased to provide increased stress ruptureproperties. Also the hafnium content is desirably slightly decreased. Arecommended alloy composition suitable for both turbine blades and vanesis therefore as follows:

Alloy Composition D Large bore gun barrels or their liners require hightensile strengths at elevated temperatures as well as high oxidationtemperatures. 'Thealloy compositions of Examples III and IV can beemployed for this purpose. Thus both of these examples exhibit superioroxidation resistance and excellent tensile-strengths at 1400 F, thealloy composition of Example III being somewhat better so far asoxidation resistance is concerned, as the oxide layer on this alloy isparticularly hard and tenacious. The alloy composition of Example IV issomewhat better so far as low temperature ductility is concerned.

As in previous examples, the tantalum content of these examples can beeliminated, and the tungsten content increased. The hafnium content isalso preferably decreased and the zirconium content increased-Also thecarbon content of Example I11 can be brought to an optimum amount ofabout 0.1%. These recommended alloy compositions for gun barrels areapproximately a follows:

The titanium content in alloy composition E and the aluminum content inalloy composition F provide enhanced oxidation resistance.

Another use of the alloy of the present invention is for soundsuppressors for jet engines. These suppressors must withstand hightensile stresses and have high oxidation reistance and also the alloyfrom which they are made must exhibit ease of fabrication. The alloycomposition of Example IV can thus be employed. The titanium and carboncontent of this alloy composition provides good fabricability and thetitanium enhances the oxidation resistance. As discussed above, thealloy having this composition exhibited a low temperature elongation of13%, yield tensile strength of 129,000 p.s.i. and an ultimate tensilestrength of 133,000 p.s.i. at room temperature and excellent oxidationresistance characteristics.

Another example of the composition of an alloy which can be employed fora sound suppressor is as follows:

Example VI Cb-Balance Hf-30% Ta-.3 6 (residual) The alloy having thiscomposition had a surface recession ranging from .002 to .004" and anoxide layer thickness of .030" when heated at 2400 F. in air for 24hours. The relative high carbon content imparted improved ductility tothe alloy. Thus the alloy of this example, when tested for elongation atroom temperature, had an elongation greater than 20% and retainsductility as determined by elongation tests of 6% and 7% after forgingand before annealing. As in the alloy of Example IV, the titaniumcontent also enhances oxidation resistance.

The tantalum content in either of these Examples V and VI serves nouseful purpose and can be eliminated and in the alloy of Example V I,the tungsten content is preferably increased. Thus two recommended alloycom-. positions for sound suppressors are as follows:

In common with other alloys for use at elevated temperatures thefabricated alloy of the present invention can be coated with primarycoatings to improve their oxidation resistance. There are a number ofcommercial coatings, most ofwhich' consist largely of silicon and whichcan be applied as a powder to heated surface areas, or applied as ,aslurry to cold surfaces and after drying heated in an inert atmosphereto about 2400 F. These coatings are of assistance in preventingoxidation of the exposed surfaces of the fabricated alloy, but goodoxidation resistance of the alloy itself is still of major importance.Thus pin holes may be present or develop in the coating or portions ofthe coating may be accidentally removed to expose the alloy to oxidizingconditions at high temperature. This can result in rapid destruction ofelements fabricated from an alloy unless the alloy itself has highoxidation resistance at such temperatures.

The alloy of the present invention can be prepared by conventionalmethods usually involving the employment of electron beam or electricarc melting, or a combination of the two, under vacuum conditions. Thealloying materials should be of high purity so far as undesired elementsare concerned, although small amounts of residual impurities willinevitably be present. 7 I

I claim: I,

1. A columbium base alloy consisting essentially by weight of about 28to 33% hafnium, 11 to 20% tungsten, 0.5 to 2% zirconium, up to about 1%aluminum, up to about 0.15% iron, up to about 0.33% carbon, 'up to abouttitanium and up to about 5% tantalum, the balance being essentiallycolumbium.

2. An alloy in accordance with claim 1 in which the amount of hafnium isabout 28 to 32%, the amount of tungsten is about 13 to 20%, the amountof zirconium is about 0.7 to 1.8%, the amount of aluminum is up to about0.5%, the amount of iron is up to about 0.15 the amount of carbon is upto about 0.15 the amount of titanium is up to about 5%, the amount oftantalum is up to about 4% and the balance is essentially columbium.

3. An alloy in accordance with claim 1 in which the amount of hafnium isbetween about 28 and 30%, the amount of tungsten is between about .13and 17%, the amount of zirconium is between about 0.7 and 1.8%, theamount of aluminum is up to about 0.25%, the amount of iron is up toabout 0.07%, the amount of carbon is up to about 0.1%, the amount oftitanium is up to about 5%, and the balance is essentially columbium.

4. The alloy in accordance with claim 3 in which the amount of hafniumis about 28%, the amount of tungsten is about 17%, the amount ofzirconium is about 1.2%, the amount of iron is about 0.07%, and thebalance is essentially columbium.

S. The alloy in accordance with claim 3 in which the amount of hafniumis about 28%, the amount of tungsten is about 15%, the amount ofzirconium is about 1.2%, the amount of carbon is about 0.1%, the amountof titanium is about 5%, and the balance is essentially columbium.

6. An alloy in accordance with claim 3 in. which the amount of hafniumis about 28%, the amount of tungsten is about 15%, the amount ofzirconium is about 1.2%, the amount of aluminum is about 0.25%, theamount of carbon is about 0.1%, and the balance is essentiallycolumbium.

7. An alloy in accordance with claim 3 in which the amount of hafnium isabout 28%, the amount of tungsten is about 17%, the amount of zirconiumis about 1.2%, the amount of carbon is about 0.1%, and the bal ance isessentially columbium. v

8. An alloy in accordance with claim 3 in which the amount of hafnium isabout 28%, the amount of tungsten is about 15%, the amount of zirconiumis about 1.8%, the amount of carbon is about 0.1%, the amount oftitanium is about 5%, and the balance is essentially columbium.

9. An alloy in accordance with claim 3 in which the amount. of hafniumis about 28%, the amount of tungsten is about 15%, the amount ofzirconium is about 1.8% the amount of aluminum is about 0.2% the amountof carbon is about 0.1%, and the balance is essentially columbium.

References Cited UNITED STATES PATENTS 3,125,445 3/1964 Lottridge l743,152,891 10/1964 Begley 75174 3,173,784 3/1965 Wlodek et a1. 75l74-3,317,314 5/1967 VVlodek et a1. 75l74 3,341,370 9/1967 Bradley et al.148-433 OTHER REFERENCES Development of Columbium and Tantalum Alloysfor Elevated-Temperature Service, Bureau of Mines Report 6558, 1964, pp.6 and 7.

CHARLES N. LOVELL, Primary Examiner

